Rapid actions of 17β-oestradiol on a subset of lactotrophs in the rat pituitary

Increasingly the role of rapid mechanisms of steroid action in physiological regulation are being recognised. We have investigated rapid effects of 17beta-oestradiol (E) on prolactin (PRL) release in vitro. Pituitary segments from male rats were incubated for 5, 10 or 20 min in Earle's balanced salt solution containing 1.2 mm tannic acid (to enable visualisation of exocytosed secretory granules by electron microscopy) either alone (control) or containing 10(-10)-10(-8) m E conjugated to bovine serum albumin (E-BSA). PRL and leuteinising hormone (LH) release from pituitary segments were also determined in response to E and E-BSA by radioiinmunoassay. Within 10 min E-BSA and E (10(-12)-10(-6) M) stimulated a significant (P < 0.05) concentration-dependent release of PRL but not LH. After exposure to experimental media for 5 min, only occasional exocytosis from type I lactotrophs (characterised by large polymorphic secretory granules) was observed in either control or E-BSA treated tissue. In contrast, E-BSA (10(-10)-10(-8) M) induced a significant (P < 0.05) increase in the number of exocytotic profiles from type II lactotrophs (characterized by smaller, spherical granules). This effect was not inhibited by removal of extracellular calcium, or by pre-treatment of cells with the RNA synthesis inhibitor actinomycin-D (0.5 mug ml(-1)), the protein synthesis inhibitor cycloheximide (1 mug ml(-1)) or the anti-oestrogen ICI 182,780 (1 muM). FACS analysis demonstrated binding of E-BSA-fluorescein isothiocyanate (FITC) (10(-10)-10(-7) M) to a subpopulation of anterior pituitary cells. The E-BSA-FITC binding sites assumed a patchy distribution across the cell surface. In conclusion, we report for the first time a rapid, non-genomic effect of E on PRL secretion in normal pituitary tissue.